This invention relates to vibration transducers for measurement of mechanical vibrations such as vibrations of machinery and, more particularly, to a vibration transducer sensitized to respond to a vibrational component along a single axis of vibration, and including dual optical sensors allowing increased bandwidth of spectral measurement while preserving good signal-to-noise ratio.
Vibration transducers are widely used for measuring vibrations, one important use of the vibration transducer being in the measurement of vibrations produced by machinery. By way of example, a turbine produces vibrations having a number of spectral components which introduce a signature to the vibration, which signature can be identified to indicate proper operation of the turbine. An output electric signal of the transducer may be applied to a spectrum analyzer for identification of specific spectral components and for measurement of the amplitude of the spectral components. The relative magnitude of the spectral components can be used to identify mechanical components, such as gears of a gear train, to determine the proper operation of the mechanical components and the detection of a fault in the operation of the turbine.
In the construction of a vibration transducer, a vibratory element having a predetermined mass is suspended on a spring having a predetermined stiffness. The mass and the stiffness determine a resonant frequency of the vibratory element. By placing the resonant frequency at a value significantly lower than the spectrum to be measured, the transducer responds to displacement of a vibrating object to which the transducer has been affixed for measurement of vibration. By placing the resonant frequency significantly above the spectrum of vibration to be measured, the transducer becomes responsive to acceleration of the vibrating object to which the transducer has been affixed for measurement of vibration. Acceleration or displacement signals produced by the transducer can be integrated or differentiated, respectively, to provide velocity information relative to vibrations of the vibrating object.
A situation of particular interest is the measurement of wide-band vibrations of the vibrating object under observation with a transducer sensitive to acceleration. In such situation, the mass of the vibrating element is minimized and the stiffness of the suspending spring is maximized to increase the resonant frequency of the vibratory element. However, the increased resonant frequency introduces a problem with such a mechanical arrangement in that the excursions in position of the vibratory element, induced by vibrations of the object under observation, are significantly reduced in amplitude from that which would be obtained by use of a larger mass in the vibratory element and softer spring in the suspension. For example, in the case of a transducer employing an optical sensor of displacement of the vibratory element, reduction in the excursion of the vibratory element reduces output signal strength from the sensor with a consequent reduction in signal-to-noise ratio and a loss of information about vibrations of the object under observation. A further problem is the introduction of cross-coupling of energies from one vibrational mode to another vibrational mode which tends to obfuscate information attainable by observation of a single selected axis of vibration.